Spectral instruments measuring e.g. infrared absorbances at several wavelengths in order to determine contents of specific components in a liquid such as milk are well known. Also X-ray analysis for determining the fat content of meat has been known for several years.
Typically such instruments apply regression analysis and multivariate calibration. Such analysis is known from e.g. the applicant's own PCT application No. WO 95/16201 disclosing the determination of extraneous water in milk samples using regression analysis and multivariate calibration. Further, the applicant's PCT application No. WO 98/43070 discloses measurement of acetone in milk using IR spectroscopy and multivariate calibration. The transfer of calibrations from one instrument to another has been discussed in U.S. Pat. No. 5,459,677 disclosing a “Calibration transfer for analytical instruments” and U.S. Pat. No. 5,559,728 disclosing “Calibration transfer for second order analytical instruments” and in the applicants U.S. Pat. No. 5,933,792 “Method of standardizing a spectrometer”.
The applicant's WO 93/06460 discloses an infrared attenuation measuring system, including data processing based on multivariate calibration techniques, and the applicants U.S. Pat. No. 5,252,829 discloses a determination of urea in milk with improved accuracy using at least part of an infrared spectrum.
As disclosed in WO 01/29557 the properties a medium of food or feed, such as the fat content of meat, may be determined by use of dual X-ray absorptiometry, the medium being a raw material of food or feed, a product or intermediary product of food or feed, or a batch, sample or section of the same, the method comprising—scanning substantially all of the medium by X-ray beams having at least two energy levels, including a low level and a high level, —detecting the X-ray beams having passed through the medium for a plurality of areas (pixels) of the medium, —for each area calculating a value, Alow, representing the absorbance in the area of the medium at the low energy level, —for each area calculating a value, Ahigh representing the absorbance in the area of the medium at the high energy level, further comprising for each area generating a plurality of values being products of the type Alown*Ahighm wherein n and m are positive and/or negative integers or zero, and predicting the properties of the medium in this area by applying a multivariate calibration model to the plurality of values, wherein the calibration model defines relations between the plurality of values and properties of the medium. The advantage over the prior art is a more accurate determination of the properties, such as the fat content in the medium. The accuracy is specifically improved over the prior art when measuring layers of varying thickness. A further advantage is due to the fact that using the described method almost the whole product is measured instead of a sample thereof. Generally, extraction of a sample from—an inhomogeneous medium will introduce an error, because the sample may not be truly representative.
Preferably the calibration model is obtained by use of a multivariate regression method being included in the group comprising Principal Component Regression (PCR), Multiple Linear Regression (MLR), Partial Least Squares (PLS) regression, and Artificial Neural Networks (ANN).
A problem related to the prior art:
It is well known, that when a number of measurement instruments measure the same sample, each instrument will generally produce an instrument specific signal if no specific actions are taken to ensure that the signals produced by the instruments are identical for an identical sample. It is equally well known that it is desirable to be able to manufacture measuring instruments, which generate the same signal when exposed to the same sample.
Calibration of an instrument may remedy the problem. However the multivariate calibration, which is applied for the DXR analysis—as described above and in the published WO 01/29557—is a delicate matter requiring a number of known reference samples, which typically have to be analyzed by an officially recognized reference method. The provision of such analysis results of the delicate perishable calibration samples consisting of various mixed samples of meat and fat which have to be handled very carefully preferably as frozen items—are time consuming, tedious and expensive. Further the calculations required for providing the calibration are time consuming too and therefore, expensive. These drawbacks are emphasized when a large number of instruments must be calibrated. Furthermore, when such calibrations must be performed often, such as regularly due to drifting in the instruments, the calibration method may strongly influence the usefulness of the instruments in a negative manner.
Thus, an object of the present invention is to provide a method, which seeks to avoid the above-mentioned drawbacks. Accordingly it is an object to disclose a method and instruments enabling a single highly sophisticated calibration developed on a master instrument to be applied to all other instruments in a series of similar instruments.
More specifically it is an object to provide a method of adjustment or correction for a series of instruments in such a manner that they can use the same calibration.
The present invention is specifically useful to measurements on meat performed by use of Dual X-ray equipment designed for measuring fat and areal density in meat, as well as for detecting foreign bodies in a meat sample. Such a measurement should, in order to obtain an acceptable accuracy, detect X-ray attenuation at at least two X-ray energies. According to the particular aspect of the present invention an X-ray equipment comprising two X-ray sources and two X-ray detectors is used for measuring the absorbances. Measurements performed with such equipment have shown to be extremely delicate as the amount of X-rays absorbed by adipose (fat) and muscle tissue only differs slightly, thereby demanding extreme care in calibrating the instrument.
Thus, another object of the present invention is to provide a method, which ease set-up of an Dual-X-ray instrument so that it is capable of producing accurate measurement of fat and areal density in meat.
A further object is to allow for less restrictive instrument specifications, enabling the use of cheaper components having rather coarse tolerances, thereby reducing the total cost of the instrument.